SOLUTION

The schematic diagram of a brushless dc motor driver module is shown in Fig. The stator (armature) and rotor are connected in three-phase. The stator is connected to a variable voltage current source through an inductor and an inverter comprising six SCRs (Si to S6). In place of SCRs, power transistors or FETs could be used according to the power rating of the motor.

Diodes are connected across SCRs to protect these from the L(di/dt) voltage induced in the armature coil undergoing commutation. Position sensors placed on the rotor provide the signal to the sensor decoders and gate drivers which cause the SCRs to be fired in sequence so as to be in synchronism with the rotor’s mechanical position. The stator and rotor fields thus get locked into each other and remain in synchronism at any rotor speed.